The purpose of this study is to develop a nonhuman primate model of infection and disease due to human T lymphotropic virus, type I (HTLV-I). HTLV-I is an important human pathogen for which no effective treatment or vaccine exists. HTLV-I is the causative agent of Adult T Cell Leukemia/Lymphoma (ATLL), and a degenerative neurological disorder known as Tropical Spastic Paraparesis/HTLV-I-Associated Myelopathy (TSP/HAM). In addition, HTLV-I is associated with the development of several autoimmune diseases in which the virus may trigger the disease process. These diseases include polymyositis, uveitis, arthritis, lymphoid interstitial pneumonitis and thyroiditis. HTLV-I is transmitted through blood and body fluids, and co-infections with human immunodeficiency virus are very frequent (up to 5% of HIV-infected individuals are co-infected). Co-infections may hasten progression to AIDS and increase the risk for development of ATLL, TSP/HAM, and autoimmune diseases. A pathogenic strain of HTLV-I, isolated from a New Orleans, Louisiana, patient, has been used to inoculate seven juvenile rhesus macaques. All seven monkeys demonstrated evidence of infection, either by seroconversion or virus expression as determined by culture or polymerase chain reaction (PCR). One animal developed polymyositis, uveitis, and arthritis 5 months following inoculation. A second animal, co-infected with simian immunodeficiency virus (SIV), developed a hematological syndrome characterized by severe anemia, thrombocytopenia, and the presence of abnormal circulating lymphocytes resembling those seen in ATLL patients. The HTLV-I macaque model therefore will be useful for studying HTLV-I disease pathogenesis and also for designing treatment algorithms.